Exchangeable sodium is increased 9 and 15% above controls at 7 and 14 days after induction of mild nephritis in rats. This must occur in a step-wise fashion since a study of daily sodium balance revealed no period of intense sodium retention. GFR is homeostatically maintained in these nephritic rats despite histologic and physiologic evidence of extensive glomerular injury. This occurs in large part due to dilatation of the afferent arteriole with subsequent increase in glomerular capillary pressures and flows. Studies are in progress to determine the relationship if any between the increase in exchangeable sodium and the homeostatic maintenance of GFR.